Centrifuge operation presents a unique set of design criteria where precision control of the rotational operation of the centrifuge is required. The wide variety of biological and chemical experimental research which use centrifugation as their primary tool to achieve component separation and perform experimental assays places a requirement of versatility on the operational characteristics which must be built into the centrifuge.
The centrifuge rotor is driven to extremely high rotational speeds in order to generate the centrifugal field required for biological research use. The high rotational speeds of the rotor cause a severe build-up of kinetic energy, which if released, can lead to destructive explosion of the centrifuge and injury or damage to its surrounding environment as well as the human operator. Centrifuge rotors typically can fail if the rotor is run in excess of the speed designed for its safe operation. The slightest imbalance of the rotor or load which it carries can cause catastrophic failure.
The centrifuge apparatus has numerous rotors which may be interchangeably used in conjunction with the same centrifuge motor and drive shaft in order that a diversity of biological experimentation may be achieved. One standard of centrifuge design encompasses an induction motor and shaft which can accommodate the interchangeable rotors to be carried on the spindle of the motor shaft, each rotor having a different weight and strength of material and a different maximum safe speed above which the particular rotor should not be operated.
Also, even the slightest imbalance of the rotor or load which it carries may grow to larger imbalances and associated forces as the rotor speed and centrifugal field increases. Often, these imbalances do not arise until the rotor has achieved very high speeds. Strain and stress within the material of the rotor exhibiting a slight fracture at rest can cause the rotor to dismember at maximum speeds approaching 100,000 revolutions per minute. The dynamic effect of any imbalancing forces cause complicated movement of the shaft upon which the rotor is suspended, such as dangerous whirls and gyrations.
It is therefore clear that a versatile centrifugation system requires: (1) a maximum safe rotor speed be identified for each rotor; (2) the operational use and control of the rotor during centrifugation be monitored and controlled; and, (3) that any imbalance be detected. If possible, the use of a single sensor and transducer system would provide accuracy and asynchronous information which may be used to control the motor speed for all varieties of rotors.
One such system is disclosed in U.S. patent application Ser. No. 605,365 filed Apr. 30, 1984 by Dennis Durbin and assigned to Beckman Instruments, Inc., the assignee of this application (now U.S. Pat. No. 4,551,715). That application entitled "Tachometer and Rotor Identification Apparatus for Centrifuges" is hereby incorporated by reference. In the disclosed specification, a method of rotor identification for each rotor and determination of the rotor's maximum safe speed is presented which relies on a single magnetically sensitive transducer to provide the necessary rotor identification and maximum safe speed information in a straightforward and precise manner.
The disclosure of U.S. Pat. No. 4,551,715 describes an improved signal processing circuit which allowed a tachometer signal and rotor identification signal to be generated from a single set of coding elements on the rotor. The Durbin application uses a circular array of magnetic coding elements on the rotor to provide the tachometer and rotor identification information. A Hall effect sensor is used to detect the magnetic orientation of permanent magnets imbedded in the base of each interchangeable rotor. In the preferred embodiment of Durbin, six magnets spaced at equal intervals around the circumference of the rotor are positioned to direct a north-oriented or south-oriented magnetic field outward from the base of the rotor for detection by the Hall effect sensor. The Hall effect sensor detects a change in magnetic reluctance as the permanent magnet rotates past the fixed sensor and induces a voltage in the sensor. A series of sharply defined pulses are generated in the conduction pathway and amplified in the Durbin system. Thus the Durbin disclosure presents an embodiment which is able to identify a rotor on the basis of a single transducer according to the combination and order in which north- and south-oriented magnets pass the Hall effect rotor. These same north and south pulses also produced a tach signal from which rotor speed can be identified. A central processing unit is used to compare rotor speed with rotor identification. Stored in the central processing unit is an information listing identifying the maximum rated speed for each individual rotor. Once the rotor is identified on the basis of a coded pulse train generated by the north- and south-oriented magnets imbedded within the rotor, the central processing unit reads the maximum rating information stored within its memory and compared these readings with the actual speed. The central processing unit also is aware of what had been programmed at the human operator keyboard for the desired acceleration and speed. The central processing unit functions to prevent the rotor from being programmed or actually operated beyond its intended rating. The magnetic transducer detection system represented a significant improvement over optical sensing systems which were subject to interference from dirt and damage, as well as inadvertent customer abuse when the wrong optical rotor disc was affixed to a rotor rated for a lower speed.
Conventionally, motor control has been achieved in centrifuges by controlling current or power to the induction motor which drives the rotor and shaft. Such current control may be disconnected or otherwise not be communicating with the centrifuge current bus. U.S. Pat. No. 4,286,203 to Ehret issuing Aug. 25, 1981 to Beckman Instruments, Inc., the common assignee with this application, discloses a Slip Frequency Control System for Variable Speed Induction Motors. This patented system controls rotor speed by maintaining a constant slip frequency regardless of motor speed. It is generally directed at normal operating conditions and drives the motor in response to an initial fixed frequency produced by relaxation oscillator 20. The system disclosed in the patent is not designed to respond to abnormal conditions such as rotor overspeed or imbalance.
What is clearly needed to insure centrifuge safety and sample integrity is a method of controlling the speed of the induction motor of a centrifuge through use of the information derived from the tach signal pulse train.
Additionally, it would be useful if imbalance detection could be effectuated to prevent imbalances which occur at high speeds from causing great destruction. The conventional art of centrifugation provides an electromechanical imbalance detector in the form of a microswitch which sits in proximity to the rotor shaft in order to detect whirling gyrations which may occur in the rotor shaft. The rotor shaft is fitted with a radially extending nub. A spring-mounted microswitch adjacent to the nub of the spindle shaft makes contact if the shaft wobbles excessively. This causes the capacitor to be discharged and triggers a digital signal at the output of a monostable multivibrator providing an active low pulse to the microprocessor. This detection system has worked fine at speeds below 30,000 r.p.m., but unfortunately is not sufficiently responsive to detect imbalance during high speed rotation.
What is clearly needed is a sensor detection system which not only identifies individual rotors and their maximum safe speeds, but also controls their speed, monitors their speed, and detects rotor imbalance.